Industrial Gear Oils: Tribological Performance and Subsurface Changes

Abstract

This study examined the tribological performance of three gear oils (Oils A, B and C), in relation to surface and microstructural changes. Oil A contains molybdenum dithiophosphate friction modifier, Oil B contains amine molybdate combined with zinc dialkyl dithiophosphate antiwear additive, while Oil C contains phosphonate and a commercial gear oil package. Following sliding tests of a hardened AISI 52100 steel ball on a spheroidized AISI 52100 steel disc, the worn surfaces were chemically studied using Raman and energy-dispersive X-ray spectroscopy. The tribological performance for each oil was different, likewise the nature of the tribofilm formed. After a 5 min sliding test, the hardness-depth profile of the worn surfaces was measured; also the cross-sectional microstructure was examined using scanning electron microscopy combined with focused ion beam preparation and transmission electron backscattered diffraction (t-EBSD) techniques. With Oil A, there was a relatively small increase in surface hardness (33% greater than that of the unworn surface), whereas with Oils B and C, the average hardness near the surface was 100% greater than that of the unworn surface. The cross-sectional microstructure using Oil A also differed from Oils B and C, which were quite similar. The result shows that with Oil A refinement of the ferrite grains spreads deeper into the material (> 10 µm), whilst with Oils B and C it was largely limited to 2-3 µm below the surface. It is concluded that the lubricant formulations and their associated tribofilms influenced the extent of deformation in the subsurface layers and consequently influenced the wear performance.

Keywords: Boundary lubrication; Gear oils; Mechanical properties; Subsurface microstructure; Surface chemistry.

Fig. 1

SEM micrographs showing the process of subsurface microstructural examination using the SEM–FIB technique

Fig. 2

Plots of friction coefficient and ECR film coverage for a Oil A, b Oil B and c Oil C

Fig. 3

Average a friction coefficient and b wear volume for the three oils after 5 and 120 min of HFRR sliding test

Fig. 4

Raman spectra obtained from surfaces lubricated by Oils A and B after 5 and 120 min (2 h). The red dot at the centre of the crosshair in the optical images shows the location each spectrum was taken from

Fig. 5

SEM images and EDX maps of wear scars lubricated by Oils A, B and C after 5 min and 2 h

Fig. 6

SEM images of wear surfaces generated from 2 h sliding test with Oil A (a, a1), Oil B (b, b1) and Oil C (c, c1)

Fig. 7

Cross section profiles of the wear scars generated from 2 h sliding tests with Oils A, B and C

Fig. 8

Variation of hardness with depth on a the unworn surface, and the worn surface after sliding test of 5 min with b Oil A, c Oil B and d Oil C

Fig. 9

Microstructures beneath a the unworn surface, and the worn surface after sliding test of 5 min with b Oil A, c Oil B and d Oil C

Fig. 10

Cross-sectional EBSD images of FIB lamella extracted from the centre of the worn surfaces: a, c inverse pole figure map for Oils A and B, respectively, b, d grain boundary superimposed on band contrast map for Oils A and B, respectively. Black lines represent high-angle GB’s (> 10°), and white lines represent low-angle GB’s (2° ≤ θ ≤ 10°)